Passenger transit can be broken down into categories based on the length of the trip as shown in FIG. 1. For short trip distances, under 2 miles, the transit modes of walking 110, and bicycling 120 are preferred: they require very little overhead (non-travel time associated with the trip, such as putting on shoes) but are very slow. For somewhat longer trips, between 2 and 200 miles, driving 130 is the preferred mode of travel. However, external factors such as rising fuel prices and increasing traffic congestion are driving down the speed of this travel mode and increasing its cost, especially relevant for trips over 100 miles. Fixed-wing air travel 140 offers high trip speeds, and is usually preferred for very long distances, over 1000 miles. However, there is a large overhead associated with this mode of travel, including travel to an airport, airport security lines, waiting for takeoff, and ground transport at the destination, all of which reduces the efficiency of traditional fixed-wing air transport for mid-range, or regional, trips between 100 and 1000 miles. As a result, there is a largely unmet need for fast, affordable regional transport 150.
Although the prior art seems to have appreciated the need for fast, affordable regional transport, the focus has mostly been on rail transit. Rail travel typically offers higher average speeds than car travel, but is constrained to operate within the bounds of fixed rail infrastructure. For regions with a dispersed population, including suburbanized regions, the cost of building rail infrastructure to connect a large percentage of the population is prohibitive.
VTOL (vertical take-off and landing) transport has also been attempted over the years for regional transport, and has clear potential advantages over rail travel. The ability to take off or land vertically enables passengers to start and finish their journey near their true origin or destination, be it an urban center or intersection of freeways. However, despite decades of attempts, successful VTOL transport remains elusive due to challenging technical obstacles. Among other things, prior art VTOL is prohibitively expensive to operate, has low flight speeds, limited ranges, is relatively fuel inefficient, and has a relatively poor safety record. This history is catalogued to some extent in the books “The Principles of Helicopters Aerodynamics”, J. G. Leishman, 2006 and “The Helicopter: Thinking Forward, Looking Back”, J. G. Leishman, 2007. Technical details about individual aircraft can be obtained from the book series “Jane's All the World's Aircraft” by referencing the appropriate volume.
These references, as well as all other extrinsic materials discussed herein, are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
In the void left by the lack of a viable regional transportation method, passengers are relegated to driving long distances in increasingly congested traffic, or flying regional fixed-wing transport aircraft and enduring long airport-related waits. There is little or no appreciation in the prior art that vertical take-off aircraft should or even could be modified to simultaneously have high flight speed, high fuel efficiency, and the ability to carry a substantial payload.
FIG. 2 shows a typical prior art tiltrotor aircraft 200 comprising a wing 202, a fuselage 204, and a first tilting rotor system 210 comprising a first rotor blade 212 and first nacelle 218 in aircraft cruise mode corresponding to a generally horizontal position of the nacelle 218. The aircraft is also equipped with a second tilting rotor system 220 on the opposite end of the wing 202. The second rotor system 220 is depicted in conversion from a horizontal position consistent with aircraft cruise mode to a vertical position consistent with helicopter mode.
In practice, nacelles 218, 228 on either side of the aircraft in prior art tiltrotors have a substantially identical tilt angle. The tilt angle 236 of a nacelle 228 is the angle 236 between the tilting nacelle axis 238 and the aircraft axis 234. In a typical tilt rotor aircraft 200, the nacelle 204 is also capable of operation in a generally vertical position used in helicopter mode flight. The nacelle 228 tilt angle 236 is usually affected using a tilt actuator and mechanism to convert from helicopter mode flight to aircraft cruise mode. A cross-shaft 206 is disposed within the wing 202 and runs between left and right nacelles 218, 228.
The aircraft of FIG. 2 has a relatively small fuselage, a relatively small wing aspect ratio, and has gimbaled rotors not capable of variable speed operation. It appears to have been optimized for the then-contemplated use of short-range transport with relatively few passengers, under which fuel efficiency and speed are not so important as vertical take-off capability and cost. There is nothing in the prior art indicating that those of ordinary skill in the art appreciated that vertical take-off aircraft should, or even could, be optimized for high fuel efficiency, while carrying a substantial payload.
The majority of prior art rotorcraft and prior art vertical takeoff aircraft are conventional helicopters. Conventional helicopters, such as the modern Sikorsky™ S-92, are severely limited in terms of cruise speed and efficiency. A conventional helicopter is lifted and propelled by the same predominantly horizontal rotor or rotor, one side of which advances into the oncoming flow, and one side of which retreats away from it. During cruise, the airspeed towards the tip the advancing rotor blade is much higher than that of the helicopter itself. It is possible for the flow near the tip blade to achieve or exceed the speed of sound, and thus produce vastly increased drag and vibration. This limits the forward speed of helicopters. Additionally, a rotor is an inefficient way to generate lift as compared to a wing, partially due to the dissymmetry of lift between advancing and retreating sides of the rotor.
A major step forward in the prior art was the tiltrotor configuration including, for example, the Bell™/Augusta™ BA609. Tilt-rotors represent a major step forward because they generate most or all of the lift necessary for cruise flight with a wing instead of rotors, which is considerably more efficient than rotor borne flight. Prior art tilt-rotors have had short, low-aspect ratio wings that were relatively thick because they had to support heavy rotor systems, which results in lower efficiency, L/D, as compared to fixed-wing aircraft.
Despite marginal increases in speed and forward flight efficiency of tilt-rotor aircraft relative to helicopters, the prior art tilt-rotors have failed to improve on the productivity (how fast one transports a payload) of conventional helicopters. This is because the ability of a modern tiltrotor to cruise up to 50% faster than a modern helicopter is almost entirely offset by its relatively higher empty weight fraction (aircraft empty weight divided by maximum hover takeoff weight, typically around 0.60-0.65 for prior art tilt-rotor) as compared to conventional helicopters.
Unless a contrary intent is apparent from the context, all ranges recited herein are inclusive of their endpoints, and open-ended ranges should be interpreted to include only commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.
The complexity, resulting high cost, aerodynamic inefficiency, poor safety record, and weight criticality of rotorcraft conspire to make them entirely uncompetitive with fixed-wing aircraft. Modern prior art rotorcraft have a productivity, the product of payload carried and speed, about 5 times lower than modern turboprop airliners, as reported in the AHS Journal paper “Rotorcraft cost too much” by Harris and Scully.
In view of the existing prior art and present state of technology, and without access to the present inventive subject matter, a person of ordinary skill in the art attempting to design an advanced future tiltrotor transport would tend to make a smaller diameter conventional rotor to minimize the rotor system weight. This would result in higher rotor disc loading in hover, and would constrain the wing to be short and thick to prevent the whirl flutter aeroelastic instability resulting from heavy conventional rotors at higher speeds. Furthermore, the flight envelope would be constrained to lower speeds because of the thick wing, the efficiency would be reduced because of the short wing, and the cruise altitude would be capped by the small wing area.
In summary, what are still needed are: (1) an appreciation that VTOL transport systems could realistically be as efficient and productive as fixed wing aircraft for regional transport of substantial payloads; and (2) technologies that could be used to implement such systems. In order to implement those goals, an aircraft would realistically need to have some or all of the following characteristics:
Tailored, efficient aerodynamics, especially including the inner wing, rotor blade and nacelle shaping, for efficient cruise flight at speeds up to Mach 0.65 (100 knots faster than the prior art Bell™ V-22);
Wing area and wing airfoil technology (expressed as an M2CL of at least 0.30-0.35) to provide for cruise flight at 35,000-41,000 feet (above most adverse weather and 10,000-16,000 feet higher than prior art tilt rotor aircraft);
Small empennage, low drag fuselage, low drag landing gear fairing and high aspect ratio wing to assist in providing a lift-to-drag ratio of 16-23 (3-4 times higher than the prior art Bell™ V-22);
Structures that support a very low aircraft empty weight fraction while sustaining rotor loads and providing the strength, stiffness, and durability of a high speed pressurized commercial transport (empty weight fractions 20-40% lower than in the non-pressurized prior art Bell™ V-22);
Mechanical systems that support the low aircraft empty weight fraction while sustaining rotor loads, and providing needed functionality;
A rotor system that provides a high cruise flight propeller efficiency at a cruise Mach number of 0.65 while also being capable of vertical takeoff; and
Low hover download to reduce the amount of engine power required for hover. (less than 5% of rotor lift vs. 11% the prior art Bell™ V-22 tilt-rotor).
Ideally, such systems would carry at least 20,000 pounds of payload, have a wing sized and dimensioned to have a maximum wing loading of between 60 and 140 pounds per square foot, have a wing aspect ratio between 10 and 22, and be capable of sustained cruise flight with the first rotor operating at a rotational speed no greater than 75% of the operational maximum rotational speed.